MOISTURE REQUIREMENTS FOR FUNGAL GROWTH

very near the fiber saturation point.²⁰The minimum relative humidity tolerated by the sapstain fungusOphiostoma piceae was 93 – 94% RH, which corresponded to a wood moisture content of 21 – 22% at 158C.²⁰Because the fiber saturation point can fall between 20% and 30% wood moisture content, there is a window of uncer- tainty of when decay can occur. Attempts to control wood moisture content within this range have shown that decay occurred at 27% MC in oriented strand board.²¹

The moisture content of green lumber ranges from as low as 30% to greater than 200%, and varies with wood specific gravity, amounts of sapwood and heartwood, and whether the wood is a softwood or hardwood.¹⁰ Prior to drying, decay can develop if lumber is exposed outdoors to moisture contents .20%.¹⁰ Use of green (unseasoned or wood that has not been air-dried or kiln-dried but allowed to dry in situ) lumber in construction can lead to mold and decay problems, as well as structural problems related to warping as the wood dries. Extended storage of lumber at moisture contents greater than 20% without drying can allow decay to develop.¹⁰ After lumber is kiln-dried or air-seasoned to the moisture Fig. 8.7. Lower limits of relative humidity to support the growth of fungi grouped in six categories: ¹⁶A—highly xerophilic; B—xerophilic; C—moderately xerophilic; D—

moderately hydrophilic;E—hydrophilic;F—highly hydrophilic. (Reprinted from Building and Environment, Vol. 34, Clarke, J. A., C. M. Johnstone, N. J. Kelly, R. C. McLean, J. A.

Anderson, N. J. Rowan, and J. E. Smith, A technique for the prediction of the conditions leading to mould growth in buildings, pages 515 – 521, copyright 1999, with permission from Elsevier.)

required for its use, what happens during storage and construction is critical. Wood lying outside on the ground exposed to rain is providing an excellent scenario for fungal colonization and growth. The potential for decay or mold to develop increases with time of exposure, as was evidenced by greater amounts of fungal colonization and greater diversity of species observed in kiln-dried southern pine after as little as 4 weeks of outdoor exposure.²²

8.5.2. Moisture Requirements for Mold Fungi

According to studies in the literature, moisture requirements for molds differ from that of most wood decay fungi; many molds can tolerate lower a_w than wood decay fungi. The optimal and minimal relative humidity conditions for growth vary for each species of mold and also vary depending on temperature and growth substrate.^15,18,23 In results complied by Viitanen and Ritschkoff,²³ the optimal growth for most molds occurred at high relative humidity (98 – 99%), at temperatures of 20 – 308C, while the minimum RH at which growth occurred was 65% at 308C for Aspergillus fumigatus. For other common indoor molds, the minimum RH needed for growth was reported as 85% forAlternariasp.,Cladosporium herbarum, andPenicillium chrysogenum; 93% forStachybotrys atra, and 80% for Aspergillus flavus.²³

In controlled studies, the water activity required for minimal growth was observed to increase significantly with a decrease in temperature.¹⁵ForPenicillium chrysogenumat 258C the minimuma_wto support growth was 0.79 while at 128C,a_w was 0.87.

The minimaa_wfor growth also varies depending on the test substrate. Growth on MEA (malt extract agar) occurred at lowera_w(0.79) compared to growth on wood- chip wallpaper where the minima was 0.84 for both Aspergillus versicolor and Penicillium spp.¹⁵ ForStachybotrys atra, the minimum a_wwas 0.93 on MEA but was much greater, 0.98, on woodchip wallpaper containing added nutrients.Stachy- botrys atrawas observed to grow on gypsum wallboard at 95% RH (aw¼0.95).¹⁸ By adding a carbon source to the surface of a substrate, Grant et al.¹⁵showed that greater nutrient availability decreased the minimal water activity required for growth. This provided evidence that organic “dirt” on a substrate will facilitate growth at lower relative humidity conditions.

Experiments to test growth conditions require highly controlled environments; it is difficult to control relative humidity conditions that are greater than 95% during experiments on mold growth or wood decay.¹¹If surfaces become cooler than the controlled air, condensation on these surfaces could lead to false-positive results for mold growth under nominal conditions that would not allow condensation.

Despite the potential difficulties, careful control of relative humidity and tempera- ture conditions can be achieved at a lower relative humidity range such as 70 – 95%.

More recently there has been considerable interest in developing mathematical models to predict mold growth based on relative humidity and temperature.^16,24,25 These models are an attempt to predict the lowest relative humidity at which

mold will grow for each temperature. Although knowledge of the lowest moisture tolerance has importance in predicting mold growth, the incidence of mold is much greater, and has much greater impact, when relative humidity is much higher, greater than 90%, or in cases where surfaces are wet because of condensation or where there is excess moisture due to leakage.

A “mold index” was developed by Hukka and Viitanen²⁴that derived two for- mulas for critical RH, one whenTis.208C and the other whenTis208C. Accord- ing to their mold index, at temperatures greater than 208C, the critical RH is always 80%, while at temperatures less than or equal to 208C critical relative humidity for mold growth is described by the formula

RHcrit¼ 0:00267T³þ0:160T²– 3:13Tþ100:0

According to their model, below 208C the minimum relative humidity required for growth increases to 82% at 108C and 88% at 58C²⁴.

The biohygrothermal model incorporates non-steady-state relative humidity and changing surface temperatures to predictions of mold growth, particularly spore ger- mination.^26,27 This model considers the moisture content of a model spore, as it exists on different substrates in equilibrium with the air, and determines the bound- ary conditions at which germination will occur. The model appears to be accurate at predicting the latent periods of spore germination and subsequent growth rates when compared with actual spores.²⁶

8.6. THE EFFECT OF CHANGING MOISTURE AND